I've driven both and opted for the 135i in the end. Don't have figures but 2nd gear in the 135i blows the 123d away. After that it's the same in all the gears.

From what you are saying then, in “real world” driving that there is not a great deal of difference between the two cars…… I expect most of us spend our time driving below 100 MPH in 3rd,4th,5th and 6th gears and from what you are saying in those gears the two cars perform pretty similarly.

Nope, I meant to say that after 2nd gear in the 135i it's a VERY noticeable difference. 1st is so closely meshed that pulling away I didn't notice much between the two. I'm still running mine in (900 miles so far) so am not really pushing the engine beyond 4.5k revs.

I will let you know more once i've done the 1200 miles, then I will be able to open her up a bit.

Nope, I meant to say that after 2nd gear in the 135i it's a VERY noticeable difference. 1st is so closely meshed that pulling away I didn't notice much between the two. I'm still running mine in (900 miles so far) so am not really pushing the engine beyond 4.5k revs.

I will let you know more once i've done the 1200 miles, then I will be able to open her up a bit.

I chewed through some numbers that I was able to find earlier, and it looked like the difference between the final drive ratios between the 135i and the 123d gives the 135i about a 9% advantage for "in gear" acceleration. The final drive ratio is 2.81:1 for the 123d, compared to the 3.08:1 for the 135i.

That's a .27:1 difference, or about a 9% difference.

Which is interesting when you look at some of the speed comparisons between the 300 ft-lb torque 135i and the 295 ft-lb torque 123d other than just the 0-60 times:

The difference in 0-100 km/h is 1.7 sec, or 24% of 7 seconds.
The difference in 0-1,000 m is 2.9 sec, or 11% of 27.3 seconds.
The difference in 80-120 km/h in 4th is .5 sec, or 9% of 5.5 seconds.
The difference in 80-120 km/h in 5th is .6 sec, or 9% of 6.6 seconds.

Note that the 123d requires TWO shifts to get to 100km/h, while the 135i only needs 1 shift. So as the differences in the number of gear shifts in the tests are minimized or eliminated, and speeds become higher, the closer the differences in acceleration approach the final drive ratio difference between the two cars.

Very interesting, eh? It makes you wonder how a 123d with a 3.08 final drive ratio would run against a 135i that is short-shifted at 5,000 rpm. With it's 144lb. weight advantage, the 123d might not be that far behind the 135i. But of course the minute the 135i runs all the way up to 7,000 rpm, it would leave even a 123d with a 3.08 final drive ratio in the dust.

The biggest performance problem for the 123d will always be the relatively low 5,000 rpm redline. BMW plays with the final drive ratio to make it drive more like it has a 5,500 rpm redline, but the higher final drive costs the 123d acceleration. And even 5,500 still counts as short-shifting in my book.

But I'm willing to live with the short-shifting and 9%+ less umph in exchange for the huge gas mileage gains and a lower price (more around the price of a 128i).

Well, by live with 9%+ less umph, I really mean I will chip the 123d, and I'll be happy to live with it if it cuts that 9%+ gap in umph down to something much less. And hopefully a chip might give me a few hundred more rpm's before redine if I'm really lucky.

I chewed through some numbers that I was able to find earlier, and it looked like the difference between the final drive ratios between the 135i and the 123d gives the 135i about a 9% advantage for "in gear" acceleration. The final drive ratio is 2.81:1 for the 123d, compared to the 3.08:1 for the 135i.

That's a .27:1 difference, or about a 9% difference.

Which is interesting when you look at some of the speed comparisons between the 300 ft-lb torque 135i and the 295 ft-lb torque 123d other than just the 0-60 times:

The difference in 0-100 km/h is 1.7 sec, or 24% of 7 seconds.
The difference in 0-1,000 m is 2.9 sec, or 11% of 27.3 seconds.
The difference in 80-120 km/h in 4th is .5 sec, or 9% of 5.5 seconds.
The difference in 80-120 km/h in 5th is .6 sec, or 9% of 6.6 seconds.

Note that the 123d requires TWO shifts to get to 100km/h, while the 135i only needs 1 shift. So as the differences in the number of gear shifts in the tests are minimized or eliminated, and speeds become higher, the closer the differences in acceleration approach the final drive ratio difference between the two cars.

Very interesting, eh? It makes you wonder how a 123d with a 3.08 final drive ratio would run against a 135i that is short-shifted at 5,000 rpm. With it's 144lb. weight advantage, the 123d might not be that far behind the 135i. But of course the minute the 135i runs all the way up to 7,000 rpm, it would leave even a 123d with a 3.08 final drive ratio in the dust.

The biggest performance problem for the 123d will always be the relatively low 5,000 rpm redline. BMW plays with the final drive ratio to make it drive more like it has a 5,500 rpm redline, but the higher final drive costs the 123d acceleration. And even 5,500 still counts as short-shifting in my book.

But I'm willing to live with the short-shifting and 9%+ less umph in exchange for the huge gas mileage gains and a lower price (more around the price of a 128i).

Well, by live with 9%+ less umph, I really mean I will chip the 123d, and I'll be happy to live with it if it cuts that 9%+ gap in umph down to something much less. And hopefully a chip might give me a few hundred more rpm's before redine if I'm really lucky.

Nice comparison.

I guess MPG figures are becoming more and more important for a lot of people these days.

Question: what happens if you come up against a chipped 135i???????

Game over, I suspect.

But got to admit, I'll eventually turn to the dark side when the eco-mentalists take over - probably opting for a 335d with the DMS chip upgrade.

I guess MPG figures are becoming more and more important for a lot of people these days.

Question: what happens if you come up against a chipped 135i???????

Game over, I suspect.

But got to admit, I'll eventually turn to the dark side when the eco-mentalists take over - probably opting for a 335d with the DMS chip upgrade.

Mad.

Chipped or not chipped, the 135i will always have the upper hand against even a chipped 123d. The point of getting a 123d and chipping it isn't to try to out-perform a 135i. The point of chipping the 123d is just to try and narrow the performance gap between the two. At the price point that I would expect the 123d to be sold at in the US, I think of it to compete more with the 128i than the 135i. The competition between a chipped 123d vs. a chipped/modded 128i would be a better comparison.

The appeal of the 123d is that it plays the balancing act between ultimate performance and rock solid fuel efficiency. Right now in the US, if you want 45+ mpg, you have to COMPLETELY give up on performance and buy a hybrid that takes around 10 seconds to reach 60 mph. Boring! On the other end of the scale, you can buy a handful of sub-6 second cars that get less than 30 mpg (many times MUCH less) for less than 35K.

The appeal of a chipped 123d for me would be the unique ability to get 45+ mpg in a 6-second car. Possibly for under 30k. There is no car for sale in the US market that offers anything near that kind of "best of both worlds" performance and fuel efficiency, especially for that price.

The 335d is nice, but it doesn't fall into those same sweet spot of 6-second performance/45+ fuel efficiency/30K price parameters. And it is not available with a manual...

Maybe I am reading the above wrong, but the 335d is a 6 second car, and isn't available as a manual, only an auto.

It also provides 36+ mpg, 0-100mph in 14secs and can easily be remapped to 340bhp (ECU is not encryted).

We picked up a 9 month old (8k miles) fully loaded 335d Coupe with 19"s for £33k, so it can be got for nearly £30k.

Sorry, I'm talking US numbers with US gallons and USD, not UK numbers with Imperial gallons and Pounds.

Over here, the 335d is rated at just 23 mpg city and 33 mpg highway according to the latest 2008 EPA test standards, with a combined mpg of only 28 US gallons per mile.

And they don't go on sale here in the states until later this year, so there are no used ones here. Even worse, BMW is expected to only sell them here in the US with a bunch of options as standard, so it is expected to be over $50,000 USD.

So while the 335d is pretty close to a 6-second car, it just doesn't come anywhere near what the 123d in projected price and MPG.

I fat fingered the thing about the manual transmission. I meant to say it was not available with a manual. I corrected that.

the following considers just the acceleration and forgets about the mpg's, residuals etc.

i drove the 123d first - purely our of curiosity. the car was very solid compared with my vw vr6 (naturally, the vw is almost 15 years old). I've never driven any bmw before and was surprised how well the car responded to driver input especially at roundabouts. The diesel felt very quick, quicker than the vr6, there was a slight delay in power, but nothing major. It did have a diesel sounding engine (i dont know if that matters to anyone here!?). In general the acceleration was very good the suspension perhaps a little harsh on the bumpy roads.

In comparison the 135i was instantly a different story, yes the car's look similar but the performance was clear - any prod of the accelerator resulted in instant acceleration in any gear, i'm totally serious, very surprising cruising in 5th (perhaps 6th at your peril) on a 30 mph road. There is a little more understeer in the 135i at the roundabouts with dsc on, i assume this is due to the heavier engine at the front of the car.

both cars were manual.

in general the manual gearbox was - not as smooth as my vw, i felt the bmw was a bit notchy.

all in all i went for the 135i - and has made my mind up to buy the car way before even phoning the dealer for a test drive (some would say reckless?!), i took delivery last friday and am very happy with my choice - a manual variant.